The present invention relates to poisoning effects during anesthesia.
During anesthesia with one of the agents desflurane, isoflurane or enflurane, it has been observed that patients can accidentally become exposed to carbon monoxide, CO, thus leading to an inadvertent CO-poisoning of the patient. Peter B. Berry et al. in “Severe Carbon Monoxide Poisoning during Desflurane Anesthesia”, Anesthesiology V 90, No. 2, February 1999, p. 613 report 36% COHb as highest CO level in blood due to this effect, i.e. 36% of hemoglobin loaded with CO (instead of oxygen) after only 15 min of anesthesia time with desflurane. A degradation of the anesthetic agent used in conjunction with Baralime or Sodalime, generally used as absorber material for CO2 in circle breathing systems, has been identified as origin of this exposure. It has been found that degradation of the agent occurs under a condition that the CO2 absorber material is too dry. Carbon monoxide, CO, has been identified as one of the degradation products.
Usually, the accidental CO exposure goes undetected, because CO is not identified or measured by the commercially available medical gas monitors. Although clinicians are aware of the potential problem, its early recognition and immediate remedy requires experience and a thorough knowledge of the behavior of the monitoring equipment used. In the above case, described by Peter B. Berry et al., the detection occurred through a sequence of strange observations, 1st the oxygen saturation of the patient decreased to 93% in spite of a fresh gas flow with 100% oxygen, 2nd the gas analyzer being set to automatic agent identification mode suddenly switched to “enflurane” in spite of the desflurane used. Only then, the clinicians suspected CO poisoning resulting from desiccation of the CO2 absorber. A blood analysis for COHb confirmed that suspicion.
The intoxication by CO occurs through the strong binding of this molecule to hemoglobin in competition to the binding of oxygen. The affinity of hemoglobin to CO, however, is 300 times stronger than to oxygen. Therefore, it is a question of the dosage of CO that determines the COHb level in blood. Harrison N. et al. in Anesthesia, Vol. 51, p 1037–1040 (1996) notes that a CO level of 0.1% for 1 h gives a COHb level of approximately 30% and evidence of moderate to severe toxicity. In the case reported by Peter B. Berry et al., the measured COHb level was 36% after 15 min of anesthesia time. It can be concluded that the CO concentration in the inhaled gas stream in his reported case must have been of the order of 0.5%.
Gas analyzers normally applied in anesthetic environments are based on gas detection by absorption measurements. Primarily, the infrared (IR) spectral region is used. The unusual behavior of the gas analyzer in the above reported case was explained by the similarity of the infrared absorption spectrum between another degradation product trifluoromethane, CHF3, and enflurane, thus leading to the erroneous identification of the anesthetic agent.
It has been speculated by Harvey J. Woehlck in “Severe Intraoperative CO Poisoning”, Anesthesiology V 90, No. 2, February 1999, p. 353 (Editorial), that a very large number of patients are at risk to be exposed to undetected CO levels, in particular the first cases in the morning or cases on anesthesia machines that are infrequently used. Also, the use of a high flow of fresh (dry) gas enhances the likelihood that the CO2 absorber material becomes desiccated and starts to break down the agent molecules.
The complete avoidance of the described problem would require strict discipline with the renewal/exchange routine of the CO2 absorber material (cf. Harvey J. Woehick et al., Reduction in the Incidence of Carbon Monoxide Exposures in Humans Undergoing General Anesthesia, Anesthesiology V87, No 2, August 1997, p. 228). However, since this strict discipline with the renewal/exchange routine appears to be hardly feasible, an early and unambiguous identification of CO gas would be desirable. The gas monitors presently used in clinics, however, are not capable of detecting CO and react only indefinitely to its presence in the breathing gas mixture and mostly provide erroneous information to the user.